Entity Framework Code First: "The path is not valid. Check the directory for the database"

If you create a new MVC application and try to go down the Code First with SQL Server Compact route, you might encounter this error when you first start the application:

The path is not valid. Check the directory for the database. [ Path = …\WebSample\App_Data\YourDatabase.sdf ]

This error might be a surprise because by default the code first approach should create the database for you if it doesn't already exist.

Whilst this is true, what it apparently won't do is create any parent folders that the database will be stored under - in this case the App_Data folder. Adding this folder is easily accomplished by right-clicking on your project and selecting Add/ASP.NET Folder/App_Data. Once you've done this, everything should work as expected.

As an aside, the reason it's getting created in the App_Data folder is because that's the default location for "DataDirectory" in a web application. You might have seen this mentioned in sample connection strings like this:

<connectionStrings>
    <add name="MyDataContext" 
        providerName="System.Data.SqlServerCe.4.0" 
        connectionString="Data Source=|DataDirectory|MyDatabase.sdf" />
</connectionStrings>

You might get this error even if you're not explicitly specifying the connection string, as a very similar connection string will be used by convention if you don't provide one.

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